Piezoelectric actuators normally comprise a plurality of piezoelectric elements arranged in a stack. Each of these elements in turn comprises a piezoceramic layer which is provided on both sides with metallic electrodes. If a voltage is applied to these electrodes, then the piezoceramic layer reacts with a lattice distortion which leads to a usable lengthwise expansion along a major axis. Since this in turn amounts to less than two parts per thousand of the layer thickness along the major axis, a correspondingly higher layer thickness of active piezoceramic must be provided in order to achieve a desired absolute lengthwise expansion. With increasing layer thickness of the piezoceramic layer within one piezoelectric element, however, the voltage necessary for the response of the piezoelectric element also rises. In order to keep this within manageable limits, the thicknesses of individual piezoelectric elements in multilayer actuators normally lie between 20 and 200 .mu.m. A piezoelectric actuator must therefore have an appropriate number of individual elements or layers for a desired lengthwise expansion.
Known piezoelectric actuators of multilayer design therefore comprise up to several hundred individual layers. These can be arranged to form a stack and, for example, can be adhesively bonded. U.S. Pat. No. 5,438,232 discloses a process for the production of multilayer actuators by bonding individual actuators with the aid of a resin. However, such a bonded stack exhibits too low a stiffness for many applications, in particular when high forces have to be transmitted using the piezoelectric actuator. Sufficiently high stiffnesses are possessed by piezoelectric actuators of monolithic multilayer design. In order to produce them, piezoceramic green films are arranged alternately with electrode material to form a stack and are sintered together. Only in this way is it to achieve a sufficiently solid composite of the individual layers in the stack. An article by H. Moilanen et al. in the journal Sensors and Actuators A , 43 (1994) 357 to 365 discloses a process for the production of a multilayer piezoelectric actuator in which both the ceramic layers and the electrode layers are produced by alternating overprinting. In this case, drying or presintering at temperatures up to 750.degree. C. is necessary at regular intervals.
An article by S. Takahashi et al. in Ferroelectrics, 1983, Vol. 90, pages 181 to 190, discloses a process for the production of a multilayer actuator which is obtained by stacking ceramic green films printed with electrode layers on one another and laminating them, and subsequent sintering of the-stack.
In the production of monolithic multilayer piezoelectric actuators, the material properties both of the piezoceramic and of the electrode material must be taken into account during the setting of the process conditions, in particular during the sintering process. Problems are posed, for example, by the optimum sintering temperature for piezoceramic, which, in order to achieve optimum grain sizes and hence optimum piezoelectric properties as a function of the composition of the piezoceramic, may lie above 1250.degree. C. At such a high sintering temperature, only platinum can be used as the electrode material. This exhibits a weak interaction with the ceramic and can be used together with most piezoceramic materials. However, the high material costs for platinum are disadvantageous, as is the limited strength at the interface between electrode and piezoceramic.
If Ag/Pd, which is cost-effective and common in multilayer capacitors, is used as the electrode material, then the sintering temperature is limited by the melting point of the alloy, which may, for example, lie below 1130.degree. C. (in the case of Ag/Pd 70/30). Hence, one is limited to piezoceramic materials whose optimum sintering temperature lies at most at the melting point of the alloy. To lower the optimum sintering temperature, such piezoceramics contain B-site dopings of typically 20 to 50 percent in relation to the lead zirconium titanate (PZT) basic material. As a disadvantage, in the case of these ceramics a lowered Curie temperature must be tolerated, which limits the maximum application temperature of the piezoelectric actuator. In addition, in the case of this material combination there has also been shown to be a limited strength in the stack at the piezoceramic/electrode interface.